Digital display systems for use with computer systems are well known. In many graphics systems employing raster scan display devices, the all points addressable or bit plane system is employed. In this system, data is laid out in a refresh store such that when it is read out for display, successive data groups from the store relate directly to successive picture elements on the display. One of the early descriptions of such a system is found in an article entitled "Computer Graphics In Color" by Peter B. Denes, which appeared in the Bell Laboratories Record, May 1976 at pages 139 through 146. Many current micro computer systems employ the all points addressable system to generate graphics displays. One example is the Personal Computer produced by International Business Machines Corporation, when incorporating a Color/Graphics adapter card or an Enhanced Graphics adapter card. Most of the known systems can be switched to provide different display definitions, including different numbers of picture elements per raster frame, different numbers of display lines, and different numbers of available colors per picture element. None of the prior systems, to Applicants' knowledge, have employed an arrangement switchable between a first mode in which data is extracted from a refresh store at one frequency and transmitted to the display at the same frequency and a second mode in which the data is extracted from the store at this frequency but transmitted to the delay device at a frequency which is an even dividend, for example half, of the extraction frequency.